The invention relates to a dynamic seal for a rotating drive shaft, such as a drive shaft used with atmospheric discharge refiners.
Atmospheric discharge disc refiners are used in both the primary and subsequent stages of processing wood chips, cooked wood pulp, sawdust/shavings, cotton, grass, and other fibrous material. Typically, these refiners have a casing enclosing at least one disc mounted on a rotating drive shaft. (More particularly, some refiners employ two discs, each mounted on its own drive shaft, while others employ a single rotating disc mounted on a drive shaft facing a stationary disc.) It is therefore necessary to seal the opening of the casing where the rotating shaft or shafts extend into the casing.
Conventionally, atmospheric discharge refiners use a xe2x80x9cstuffing boxxe2x80x9d or xe2x80x9cpacking boxxe2x80x9d type seal for this purpose. The xe2x80x9cstuffing boxxe2x80x9d seal typically has a shaft sleeve to prevent wear and corrosion of the drive shaft, and a housing designed to hold braided-yam packing rings of various materials in groups of two to six per set. These packing rings are contained concentrically around the sleeve and assembled into the stuffing box consecutively, usually with a xe2x80x9clantern ringxe2x80x9d to distribute water or grease as a lubricant for the seal. Alternately, some atmospheric discharge refiners employ mechanical seals, such as those used in boiler feed water pumps. These mechanical seals typically have hardened seal rings that rub against carbon rings, which are cooled and lubricated by high-pressure water supplied by a dedicated booster pump system.
While both types of seals have been employed with atmospheric discharge disc refiners, neither has proved satisfactory. The packing rings of the xe2x80x9cstuffing boxxe2x80x9d type seal must be compressed radially against the rotating shaft sleeve to prevent fiber, water, steam and other process media from escaping the casing and causing spillage and damage to the refiners"" bearings. The compression device that provides this compression (e.g., a xe2x80x9cgland-follower xe2x80x9d) must be adjusted by way of two, three or four take-up nuts and studs, or bolts. The packing compression of the packing rings must be monitored and adjusted at regular intervals for proper performance of the seal, requiring maintenance personnel""s physical attention at least daily.
Further, misalignment or improper tightening of the gland follower can cause the packing rings to overheat and burn up from friction. Additionally, improper adjustments can allow leaks and spillage of the process media from the casing, contaminating the packing and causing packing ring failure and premature wear of the shaft sleeve. Still further, the assembly of the xe2x80x9cstuffing boxxe2x80x9d type seal requires large amounts of axial shaft space, making it difficult to reach for service and adjustment. Moreover, the seal""s excessive use of the space along the shaft may decrease the amount of space that can be used for feeding the fiber stock into the casing. As a result of these problems, xe2x80x9cstuffing boxxe2x80x9d type seals are seldom maintained properly, and a large number of refiner operating failures and shortened operating cycles for refiners are caused by the use of this type of seal.
With regard to mechanical seals, these seals are typically not practical for atmospheric discharge refiners. Mechanical seals have very tight running tolerances, and cannot be axially adjusted as required for atmospheric discharge refiner applications. Further, these seals typically cannot perform well with the operating vibration levels of atmospheric discharge refiners. In addition, the costs of such mechanical seals and the adapters necessary to employ them on refiners are prohibitive, and maintenance or replacement of mechanical seals requires disassembly of the main rotating element of the refiner, leading to a loss of production and high maintenance expense.
Accordingly, there is a need for an improved drive shaft seal for atmospheric discharge refiners. The dynamic seal according to the invention advantageously offers a drive shaft seal that can effectively and efficiently seal an atmospheric discharge refiner. The seal according to one embodiment of the invention includes a seal housing with at least one inlet for injecting a fluid (i.e., a liquid or gas) into the seal. This embodiment also employs an expeller device, for expelling the injected fluid along the drive shaft in the direction of the casing, so as to provide a dynamic seal for containing the process media or fiber stock within the casing. In particular, the seal may include an expeller ring mounted on the shaft such that the expeller ring rotates with the shaft to expel the injected fluid from the seal along the shaft toward the casing.
In addition, the seal according to various embodiments of the invention may employ seating rings positioned to prevent the injected fluid from escaping the seal in a direction other than along the shaft toward the casing. For example, some embodiments of the invention may employ a glass-filled polytetrafluoroethylene ring positioned proximal to the expeller device. Various embodiments of the invention may also employ a seal adjuster for compressing a graphite-yarn packing ring against the glass-filled polytetrafluoroethylene seal ring. Still further, the seal housing may include an internal flange to create a chamber for containing the fluid injected into the seal. Additionally, the seal may include a fitted sleeve covering the drive shaft to protect the shaft from wear and corrosion.
The dynamic seal according to the invention advantageously provides a seal that effectively and efficiently seals the entrance of the drive shaft into the casing, to prevent process media from escaping the casing. Further, the seal according to the invention is substantially smaller than conventional xe2x80x9cstuffing boxxe2x80x9d type seals, allowing additional space for feeding the process media into the container. Also, the seal according to the invention can be easily adjusted to provide movement along the axial length of the drive shaft when the axial position of the disc is changed.